Why C?

1
Todays Material

• Why C?
• History of C
• Generic C Program Structure
• Variables
• Type and Name
• Assignment Operator
• Simple I/O
• scanf/printf

2
Why C?

• Simple and Structural
• Minimal syntax
• Availability and Portability
• C compilers are available for a wide range of
  platforms
• Windows, UNIX, embedded systems
• Language of choice for system programming
• Most OS code are written in C
• Widely known
• Almost every programmer knows C
• There are lots of public code written in C

3
History of C

• C evolved as follows
• CPL Combined Programming Language (Barron, 1963)
• BCPL Basic CPL (Richards, 1969)
• B (Thompson, 1970)
• C (Kernighan Ritchie, 1973)
• ANSI C American National Standards Institute C
  (X3J11, 1989)

!! The fact that a language originated in 1973
has required as little change as this one has in
thirty years of heavy use is truly remarkable,
and without parallels anywhere else in computer
science or engineering.
4
Generic C Program Structure

• include ltstdio.hgt
• / main designates the starting place of our
  program /
• main()
• / Variables hold Data Items Needed in the
  Program /
• Variable Declarations
• / Steps of our program I/O, computation
  (expressions) /
• Statement1
• Statement2
• StatementN
• / end-of-the program /

5
First C Program
PROBLEM Convert a fahrenheit
temperature to celsius

• include ltstdio.hgt
• / Convert fahrenheit to celsius /
• main()
• float fahrenheit
• float celsius
• printf(Enter a temp in fahrenheit )
• scanf(f, fahrenheit)
• celsius (fahrenheit-32)/1.8
• printf(f degrees fahrenheit equals f degrees
  celsius\n, fahrenheit, celsius)

6
First C Program Dissected

• include ltstdio.hgt
• / Convert fahrenheit to celsius /
• main()
• float fahrenheit
• float celsius
• printf(Enter a temp in fahrenheit )
• scanf(f, fahrenheit)
• celsius (fahrenheit-32)/1.8
• printf(f degrees fahrenheit equals f degrees
  celsius\n, fahrenheit, celsius)

Load the standard library to handle I/O
Comments In between / . /
main() Designates where the execution will start
Curly braces Determines the beginning and the end
of a code block
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First C Program Dissected (cont)

• include ltstdio.hgt
• / Convert fahrenheit to celsius /
• main()
• float fahrenheit
• float celsius
• printf(Enter a temp in fahrenheit )
• scanf(f, fahrenheit)
• celsius (fahrenheit-32)/1.8
• printf(f degrees fahrenheit equals f degrees
  celsius\n, fahrenheit, celsius)

• Variables
• A variable is a memory location whose contents
  can be filled and changed during program
  execution
• float is the variable type
• fahrenheit celsius are variable names

• Statements
• Steps of your program.
• Statements end with a semicolon ()

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First C Program Dissected (cont)

• include ltstdio.hgt
• / Convert fahrenheit to celsius /
• main()
• float fahrenheit
• float celsius
• printf(Enter a temp in fahrenheit )
• scanf(f, fahrenheit)
• celsius (fahrenheit-32)/1.8
• printf(f degrees fahrenheit equals f degrees
  celsius\n, fahrenheit, celsius)

Output Use printf function to print something on
the screen
Input Use scanf function to read something from
the keyboard
Computation Use mathematical operators to perform
computation
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From C Code-2-Machine Code

• Now that we have our C program, how does it get
  translated to machine code, i.e., to 0s and 1s
• Remember a computer understands just 0s and 1s
• We use several system software components to
  translate out C program to machine code
• Compiler, assembler, linker, loaderOS

10
First C Program Executed
PROGRAM

• include ltstdio.hgt
• / Convert fahrenheit to celsius /
• main()
• float fahrenheit, celsius
• printf(Enter a temp in fahrenheit )
• scanf(f, fahrenheit)
• celsius (fahrenheit-32)/1.8
• printf(f degrees fahrenheit equals f degrees
  celsius\n, fahrenheit, celsius)
• / end-main /

DATA
fahrenheit
75.4
?
?
celsius
?
24.11
Enter a temp in fahrenheit 75.4 degrees
fahrenheit equals 24.11 degrees celsius
75.4
11
Whats a Variable?

• A memory location whose contents can be filled
  and changed during program execution
• Each variable has a type and name
• Type defines the type of information that can be
  stored in that memory location
• Name is a label that allows us to refer to that
  memory location

Memory
name

• int number1 / stores a natural number
  (152) /
• float number2 / stores a real number
  (65.324) /

12
Variable Declaration

• The basic format for declaring variables is
• datatype varName, varName, ...
• where datatype may be
• int / Stores a natural number, e.g.,
  34532 /
• float / Stores a real number, e.g., 15.342
  /
• double / Also stores a real number, but more
  precision /
• / e.g., 345.22359573943 /
• char / Stores an ASCII char, e.g., A /

13
Variable (Identifier) Names

• C identifiers consist of letters and digits in
  any order, except that
• The first character must be a letter
• fahrenheit, celsius, sum, a123, i1, i2, i3
• The identifier can be in lowercase or uppercase
• The upper and lower cases may be mixed
• sum, Sum, SUM
• The underscore (_) can be included and it can
  also be the first char of the identifier
• total_sum, num_students, _localVar

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Variable (Identifier) Names (cont)

• C identifiers consist of letters and digits in
  any order, except that
• The identifiers should not contain a blank space,
  hyphen or quotes
• sum quotes() is illegal
• total-sum illegal character ( - )
• total sum blank space should not be there
• Identifier names are case-sensitive
• sum, Sum and sUm are all different identifiers

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Variable Examples Assignment
int x / Define 1 int / float fahrenheit,
celsius / Define 2 floats / double d /
Define 1 double / x -85 / Assign -85 to
x / d 3.4545673 / Assign 3.4545673 to d
/ fahrenheit 75.0 / Assign 75.0 to
fahrenheit / celsius 23.4 / Assign 23.4
to celsius /

• is the assignment operator
• Syntax identifier value
• Changes the contents of the memory location that
  the variable refers to

75.0
3.4545673
-85
23.4
x
fahrenheit
d
celsius
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Variable Declaration Initialization

• Variables can be initialized during declaration

int sum 32000 / can be positive / int x
-23, y 458 / and negative / float f1
34.5 float fahrenheit 75.3, celsius
0 double d1, d2 -4.567 / Can be negative
/ double d3 5e3 / Scientific notation
5x103 / d1 675e-4 / 675x10-4 0.0675 /
32000
-23
458
34.5
75.3
0.0
0.0675
-4.567
5000
sum
x
y
f1
celsius
fahrenheit
d1
d2
d3
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Character Variables

• Represents a single character
• Characters are letters of the alphabet (both
  upper and lower case)
• Ten digits 0 through 9
• Special symbols such as . , - !
• Characters need to be enclosed in single quotes
• e.g. 'A'

char letter letter 'A' / Letter A /
letter 9 / Digit 9 /
char letter c / initialization /
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Character Variables (more)

• In fact, char corresponds to 1 byte natural
  number
• char type variables occupy 1 byte in memory
• Whats really stored in a char variable is the
  ASCII value of the character
• ASCII value of A is 65
• ASCII value of B is 66
• ASCII value of 0 is 48
• ASCII value of 1 is 49
• http//www.asciitable.com/

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ASCII Table
20
Special Characters

• Characters are enclosed in single quotes
• How do I denote a single quote?

?
char letter letter '''
?
char letter letter '\''

• When a backslash (\) is used in front of a
  character, the combination is called an Escape
  Sequence

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Escape Sequences

• Combinations of a backslash (\) and a character
  that tells the compiler to escape from the ways
  these character would normally be interpreted
• Commonly used escape sequences
• \n move to next line
• \t move to next tab setting
• \\ backslash character
• \' single quote
• \" double quote

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sizeof Operator

• You can learn the number of bytes that a certain
  type occupies in memory using the sizeof operator

int a sizeof(char) / Returns 1 / int b
sizeof(short) / Returns 2 / int c
sizeof(int) / Returns 4 / int d
sizeof(long) / Returns 4 or 8 / int e
sizeof(float) / Returns 4 / int f
sizeof(double) / Returns 8 /
1
2
4
8
4
4/8
a
b
e
f
c
d
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Keyboard Input/ Display Output

• Achieved using scanf/printf functions
• Definitions of these functions are in ltstdio.hgt
• So we must include these definitions at the start

include ltstdio.hgt int main() scanf() printf
() / end-main /
24
printf/scanf Functions

• Formatted input/output functions in C
• Definitions in standard I/O library ltstdio.hgt
• If we are using these functions in our programs,
  we must include these definitions by include
  ltstdio.hgt at the beginning of our programs

printf(FormatString, expression1, expression2,
) scanf(FormatString, variable1, variable2,
)

• FormatString is enclosed in double quotes
• abcdxyz
• number is d\n
• Format specification d is used to print an int

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printf Examples
int x 45 float y 56.7 printf(Whats
up?\n) / Print a msg only / printf(Number
x d, 2 times y f\n, x, y) printf(Sum
of x and y is f\n, xy)
Expression section
Format section
Whats up? Number x 45, 2 times y 56.70000 Sum
of x and y is 101.7000
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printf Format Specifiers
Character Type Output Format
c char A single-byte character
d int Signed decimal integer
f float Signed value having the form dddd.dddd, where dddd is one or more decimal digits
lf double Signed value having the form dddd.dddd, where dddd is one or more decimal digits
e float double Signed real number having the form d.dddd e signddd
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printf Examples
printf(First line.\nSecond line. Percent char
XYZ\n) printf(3rd line. Slash \\, single quote
\\n) printf(4th line. Double quote \.
End\n) printf(Line 5. char A c, ASCII value
d\n, A, A) printf(Line 6.
Tab\tTab.\n) printf(Line 7. Carriage
return\rOK\n) printf(Line 8.\n)
First line. Second line. Percent char XYZ 3rd
line. Slash \\, single quote 4th line. Double
quote . End Line 5. char A A, ASCII value
65 Line 6. Tab Tab. OKne 7. Carriage return Line
8.
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printf Examples
char c1 A, c2 c, c3 9, c4 \\ int
x 11, y 16 float f 56.7 double d
456.789345 printf(c1 is ltcgt, c2 is ltcgt, c3
ltcgt, c4 ltcgt\n, c1, c2, c3, c4)
printf(x is ltdgt, y is ltdgt, avg is ltdgt\n, x,
y, (xy)/2) printf(Sum of ltfgt and ltlfgt is
ltlfgt\n, f, d, fd)
c1 is ltAgt, c2 is ltcgt, c3 lt9gt, c4 lt\gt x is lt11gt,
y is lt16gt, avg is lt13gt Sum of lt56.70000gt and
lt456.7893450000000gt is lt513.4893450000000gt
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printf Specifying Field Length Justification
char c1 A, c2 c int x 11, y
16 float f 56.7 double d 456.789345 printf
(--------------\n) printf(7c-7c\n,
c1, c2) printf(-7d7d\n, x,
y) printf(7.2f-7.1lf\n, f,
d) Printf(--------------\n)
-------------- Ac 11
16 56.70456.7 --------------
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More printf Examples
printf(".9f\n", 300.00145678901f) printf(".19lf
\n", 300.0014567890123456789)
300.001464844 300.0014567890123700000
float has 7 digit precision here (3000014)
double has 16 digits (3000014567890123)
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scanf Examples
char c int x float y double d scanf(d,
x) / Read 1 int / scanf(c, c) /
Read 1 char / scanf(f, y) / Read 1 float
/ scanf(lf, d) / Read 1 double / / Read
1 int, 1 float, 1 double / scanf(dflf, x,
y, d)

• c char
• d int
• f float
• lf double

Variable Address
Format section
32
getchar and putchar Functions

• getchar reads a single character from the
  keyboard
• putchar writes a single character on the screen
• Example

char c printf(What-to-do Menu
\n) printf( (a) To write a C program
\n) printf( (b) To go swimming
\n) printf( (c) To watch TV \n) printf(Selec
t one option ) c getchar() / Read the
user choice / getchar() / Skip newline
\n char / putchar(B) / Prints B on the
screen / c Z putchar(c) / prints Z on
the screen /